The present invention relates to a circuit for the control of a stepping motor in battery-operated instruments, in particular watches, wherein it is desirable to keep power consumption to a minimum to thereby prolong battery life. A circuit of this type comprises an oscillator circuit, a divider circuit, and a control, or drive, circuit to generate pulses to actuate the field coil of the stepping motor. A measuring device detects the current through the field coil of the stepping motor, and a detection circuit detects the variation over time of the current through the field coil, so that the pulses generated by the control circuit to actuate the field coil of the stepping motor may be terminated upon the occurrence of a predetermined variation of the current. A first circuit determines that the duration of the pulses for actuation of the field coil is not less than a first predetermined period of time, and a second circuit determines that the duration of the pulses for actuation of the field coil does not exceed a second predetermined period of time.
A pulse drive circuit for the actuation of a single phase rotating stepping motor is disclosed in German Offenlegungsschrift No. 2,346,975. This circuit is responsive to the current flow characteristic through the field coil of the stepping motor upon the application of a rectangular control signal. This current flow depends on (a) the type of stepping motor in use, (b) the applied voltage as a function of the state of charge of the battery, and (c) the load on the stepping motor. With a certain combination of these parameters, it is possible that the interruption of the control pulses necessary for economy in power consumption will not take place, because the minimum current required for actuation of the control circuit will not be attained. In such a case, the longest possible control signal obtainable is used, even though it may not be necessary. In order to obtain energy savings in such a case, other criteria must be employed to control the duration of the driving pulses for the stepping motor, further complicating the control circuitry.
A further disadvantage of the circuit disclosed in this publication is the fact that the circuit employs a differentiating element, consisting of a resistor and a condenser, for detecting the minimum current. The capacitance needed for this circuit is difficult to provide in integrated form, and it must therefore be incorporated as an external discrete element. Furthermore, the circuit operates in an analog mode with very low voltages in the area of current measurement and detection, so that it is susceptible to interference by external radiation.